Isamu is a young Trainer from Pallet Town who received a Pokédex from Professor Oak and was sent on a quest to find out more about Pokémon.

Isamu encounters a violent male Clefairy on his way to Professor Oak's lab, and together they defeat a giant Nidoran by accident. When Clefairy follows Isamu to the laboratory, Isamu chooses to travel with the Clefairy because he finds his antics amusing.

Unlike his rival, Isamu doesn't catch a lot of Pokémon. He has two (later three) favorites, which he keeps outside their Poké Balls. He has caught several others, including Articuno, but never uses them in battle.

He continues his journey in Pokémon Ruby-Sapphire, the sequel to Pokémon Pocket Monsters that takes place in Hoenn, and in Pocket Monsters DP, the second sequel of Pokémon Pocket Monsters that takes place in Sinnoh.